Barrett's esophagus (BE) is a metaplastic condition that is associated with chronic gastroesophageal reflux disease (GERD). BE is important because it can progress to esophageal adenocarcinoma, a cancer with a high mortality rate and a rapidly rising incidence. Since identification of BE with dysplasia or early cancer can lead to cure, gastroenterological societies recommend that patients with GERD be screened by upper endoscopy Annually, ~720,000 Americans are endoscopically screened for BE, resulting in nearly a $1B cost to our health care system, a number that would increase significantly if all patients at risk for BE were screened. Nearly half of this cost is a consequence of patient sedation during the procedure. Another limitation of endoscopy is poor diagnostic performance, as recent studies have shown that this procedure has an accuracy of less than 50% for identifying BE. These shortcomings of upper endoscopy make it a good target for improvement. In the prior funding period we have developed a new technology called optical frequency domain imaging (OFDI), that is capable of acquiring depth-resolved microscopic image data at very high speeds in vivo. Using a balloon-centering optical catheter, we have demonstrated in approximately 100 patients that OFDI can safely acquire detailed three- dimensional images of the entire distal esophagus in a few minutes and that the sensitivity of this technique may be equivalent to or possibly better than that of endoscopy. Since our studies have focused on assessing feasibility and diagnostic potential, we have deployed OFDI as an adjunct to endoscopy under conscious sedation. In this proposal we will advance balloon-catheter OFDI technology so that it can be used for BE screening without sedation. We will accomplish this task by increasing the speed of OFDI by approximately a factor of 10 and by developing and comparing two novel balloon-catheter devices: a transnasal probe and a tethered capsule that can be swallowed. We will then determine the sensitivity and specificity of unsedated balloon catheter OFDI for BE diagnosis using a laser marking method to register OFDI images to biopsy. Finally, we will conduct a clinical study in 300 patients with GERD to test the hypothesis that unsedated balloon-catheter OFDI is more sensitive than endoscopy. In the end, we will have developed and validated an improved screening tool for BE that is less expensive and more accurate than the current standard of care.